Method of producing a crown wheel

ABSTRACT

Method of producing a conical or cup-shaped crown wheel which can mesh with a cylindrical pinion with the inclusion of an angle α. The crown wheel is produced by means of a generating process. The tool (1, 43) consists of a disc (3) which has machining elements (53) disposed on its circumference whose cutting edges lie in the surface of a profile which extends in the form of helical ribs (4) at a pitch angle γ around the circumference of the disc (3), said profile in each cross-sectional plane perpendicular to the helical direction of the ribs (4) being the shape of a number of adjacent cuttting teeth (53), based on the basic geometry of a cylindrical pinion which is characteristic of the crown wheel and the center point of which lies on a circle which is in a plane perpendicular to the rotation shaft of the tool, and has a center point lying on the rotation shaft of the tool. During the generating process the tool (1, 43) is positioned in such a way that the center point (5, 47) of the cutting teeth (53) which are in engagement with the workpiece (20, 38) lies essentially in the plane which is formed by the axis of rotation (22, 39) of the workpiece and the center point of the tool (1, 43) lying on the rotation shaft (2, 46) of the tool and said center point (5, 47) is moved in a straight line which intersects or crosses the axis of rotation (22, 39) of the workpiece (20, 38) with the inclusion of the angle α.

The present invention relates to a method of producing a conical orcup-shaped crown wheel which can mesh with a cylindrical pinion whoseaxis of rotation intersects or crosses the axis of rotation of the crownwheel with the inclusion of an angle α deviating from 90°, the crownwheel being produced, starting from a workpiece, by means of agenerating process in which a tool and the workpiece rotate at aconstant ratio of rotation speeds and are moved relative to each otherin such a manner that the tool continuously works the workpiece.

For a proper understanding of the present invention, it is pointed outthat a crown wheel which can mesh with a cylindrical pinion in a mutualposition of the axes of rotation in which the angle α lies between 0°and 90° is referred to as a conical crown wheel, and if the angle α liesbetween 90° and 180° , it is referred to as a cup-shaped crown wheel.

A generating process for producing conical and cup-shaped crown wheelsis known from U.S. Pat. No. 3,184,988. In the case of this known methodthe tool used is a hobbing cutter whose working profile, viewed in thedirection of the axis of the cutter, is a rack profile, which is alsousual in the case of tools for working involute cylindrical pinions. Asa result of the hobbing cutter used, the known method produces crownwheels whose teeth are characterized by barrel-shaped tooth flanks.Crown wheels with barrel-shaped tooth flanks have the disadvantage thatthey can be loaded only to a limited degree, through the fact that thetooth flanks do not always come into contact over the entire tooth widthwith the cylindrical pinion which meshes with the crown wheel.

For the production of crown wheels, it is preferable to use a hobbingcutter of the type described in the publication WO 91/04819. However,the way in which the workpiece from which the crown wheel is producedand the tool have to be positioned and moved relative to each other foraccurate production of a conical or cup-shaped crown wheel with thedesired teeth is not known from the prior art.

The object of the present invention is therefore to propose measureswhich provide a method for producing improved conical or cup-shapedcrown wheels.

This object is achieved by a method of the type mentioned in theintroduction which is characterized by the characterizing part ofclaim 1. According to this measure, the tool is positioned relative tothe workpiece in such a manner that the rotation shaft of the tool runsparallel to the surface of the part of the workpiece to be worked. Thismeans that the tool is symmetrically loaded, and the wear on the tool isminimal. The method proposed in claim 1 can advantageously be used forproducing a crown wheel which can mesh with a cylindrical pinion in thecase of which the axes of rotation of the crown wheel and of thecylindrical pinion intersect each other.

The present invention also provides a method of the type mentioned inthe introduction, which is characterized by the characterizing part ofclaim 2. This method is particularly advantageous for the production ofa crown wheel which can mesh with a cylindrical pinion in the case ofwhich the axes of rotation of the crown wheel and of the cylindricalpinion cross each other. This situation is generally known as"off-centre". The method according to claim 2 is simple to carry out andleads to accurate results. The symmetrical loading of the tool is notachieved for all points of the tooth width of the crown wheel, but onaverage uniform wear still occurs.

The measure according to claim 3 describes the way in which the desiredpositioning of the tool can be achieved on a universal gear hobbingmachine, while the feed motion of the tool is described in claim 4.

Claims 5, 6 and 7 describe the exact values of the envisaged settings.

The method according to the present invention will be explained ingreater detail below with reference to the drawing, in which:

FIG. 1 shows a diagrammatic representation from above of the movement ofthe tool relative to the crown wheel to be produced, which crown wheelcan mesh with a pinion whose axis of rotation intersects the axis ofrotation of the crown wheel;

FIG. 2 shows a representation of the position of the tool in a view inthe direction of arrow II in FIG. 1;

FIG. 3 shows a diagrammatic section along the line III--III in FIG. 1;

FIG. 4 shows a diagrammatic side view of a device for carrying out themethod according to the invention;

FIG. 5 shows a diagrammatic top view of the device in FIG. 4;

FIG. 6 shows a diagrammatic view of the production, by means of ahobbing cutter, of a conical crown wheel which can mesh with acylindrical pinion, in the case of which the axes of rotation of thecrown wheel and the cylindrical pinion cross each other;

FIG. 7 shows a section of the hobbing cutter in FIG. 6, in which theteeth are shown in the way in which they engage with the outer surfaceof the conical crown wheel;

FIG. 8 shows the same operation as that in FIG. 6, in the case of whichthe hobbing cutter is held over the entire width of the tooth at rightangles to the surface of the workpiece; and

FIG. 9 shows the same operation as that in FIG. 6, in the case of whichthe hobbing cutter is moved in one plane parallel to the axis ofrotation of the workpiece.

FIGS. 1, 2 and 3 show a cutter 1 for the production of a crown wheelfrom a workpiece 20 using the method according to the invention. Thecrown wheel to be produced in this example is a conical crown wheelwhich can mesh with a cylindrical pinion with spur toothing, with theinclusion of an angle a between the intersecting axes of rotation of thecylindrical pinion and of the crown wheel, the angle α lying between 0°and 90°.

The cutter 1, which is shown only diagrammatically in the drawing,consists of a disc 3 which is rotatable about its rotation shaft 2 andhas machining elements disposed on its circumference whose cutting edgeslie in the surface of a profile which extends essentially in the form ofhelical ribs 4 at a pitch angle γ around the circumference of the disc3. In this case, said profile in each cross-sectional planeperpendicular to the helical direction of the ribs 4 is the shape of anumber of adjacent cutting teeth, which profile shape is based on thebasic geometry of a cylindrical pinion having involute teeth which ischaracteristic of the crown wheel to be produced, the centre of whosebase circle lies on a circle which is in a plane perpendicular to therotation shaft 2 of the cutter, and has a centre point 5 lying on therotation shaft 2 of the cutter. Instead of the cutter 1, a grinding toolcorresponding to the above description could also be used for carryingout the method.

The envisaged teeth are provided in a tooth region 21 of the workpiece20 by means of a generating process. In this case the cutter 1 rotatesabout its rotation shaft 2 in the direction of arrow A, and theworkpiece 20 rotates about its axis of rotation 22 in the direction ofarrow B. The ratio of the rotation speeds of the cutter 1 and theworkpiece 20 is constant during the generating process, and the cutter 1works continuously on the workpiece 20.

During the generating process, the centre point 5 on the rotation shaft2 of the cutter 1 is moved relative to the workpiece 20 in a first planeV which comprises the axis of rotation 22 of the workpiece 20. The firstplane V forms an angle β with a second plane W, which likewise comprisesthe axis of rotation 22 of the workpiece 20. The second plane W forms aright angle (90°) with the rotation shaft 2 of the tool 1.

FIG. 2 shows clearly that the rotation shaft 2 of the cutter 1 forms anangle δ with a plane U extending at right angles to the axis of rotation22 of the workpiece 20.

FIG. 3 shows that the centre point 5 of the cutter 1 is moved along aline p in the plane V, which line forms an angle equal to the angle αwith the axis of rotation 22 of the workpiece 20.

In order to produce the crown wheel, the tool 1 is moved relative to theworkpiece 20 in such a manner that the following applies for the angle βbetween the first plane V and the second plane W: β=arctg (-sin α·tgγ)·Prior to the generating process, the angle δ is set in such a mannerthat the following applies: δ=arctg (cos α·tg γ·cos β).

The description given above is based on a crown wheel which can meshwith a cylindrical pinion with spur toothing. In that case, at the placewhere the cutter 1 is in engagement with the workpiece 20, the helicalribs 4 of the cutter 1 lie in principle parallel to the plane V.However, the present method can also be used to produce a crown wheelwhich can mesh with a cylindrical pinion with helical toothing. In thiscase then, at the place where the cutter 1 is in engagement with theworkpiece 20, an angle corresponding to the tooth angle of thecharacterizing cylindrical pinion is present between the helical ribs 4of the cutter 1, on the one hand, and the plane V, on the other.

FIGS. 4 and 5 show an example of a device which is suitable forproducing crown wheels using the method according to the invention. Thedevice comprises a miller (milling machine) 31 having a bedplate 32. Aturntable 33, on which the workpiece 20 can be fixed, is mounted on thebedplate 32. The miller 31 is provided with a drive (not shown), inorder to rotate the turntable 33 about axis of rotation 34. Theworkpiece 20 is placed on the turntable 33 in such a manner that theaxis of rotation 22 of the workpiece 20 coincides with the axis ofrotation 34 of the turntable 33. The miller 31 furthermore comprises acolumn 35 which can be moved in the direction of arrow X by means of adrive which is not shown. A milling head 36 is attached to the column35, which milling head 36 can be tilted relative to the column 35 aboutan axis 37. The milling head 36 can also be moved relative to the column35 in the direction of the arrow Y and the arrow Z. The rotation shaft 2of the tool 1 is rotatably mounted in the milling head 36 by both itsends. The rotation shaft 2 can be driven in the direction of arrow A.Using such a device, a crown wheel can be produced according to themethod described by reference to FIGS. 1, 2 and 3 by means of the cutter1, or using a tool corresponding to the description given thereof.

In another embodiment of the method according to the invention, thefollowing applies to the angle δ: δ=γ·cos α, and the following to theangle β: β=arctg (-sin α·Tg γ). This embodiment simplifies thecalculation of the value of δ and gives satisfying results in practicein respect of the tooth shape of the crown wheel.

FIG. 6 shows a part of a workpiece 38 to be worked to a conical crownwheel, and having an axis of rotation 22. The outer surface 40 of theworkpiece 38 is to be provided with the desired teeth. The crown wheelto be produced in this way can mesh with a cylindrical pinion (notshown), the axis of rotation 41 of which crosses the axis of rotation 22of the crown wheel at the distance 42. For the provision of the teeth onthe workpiece 38 in the manner according to the present invention, theworkpiece 38 is worked with a hobbing cutter 43. The hobbing cutter 43is of the type which is described with reference to FIGS. 1 -3. Thecutter 43 is suitably positioned, in order to ensure that the spirallyplaced teeth on the outer surface of the cutter 43 at the position ofthe engagement region 48 with the workpiece 38 run in the direction ofthe teeth of the cylindrical pinion (not shown). Achieving the desiredpositioning and the effect envisaged thereby is explained in conjunctionwith FIG. 7.

FIG. 7 shows a section of a part of the cutter 43. The cutter 43 hascutter teeth 53 which have a profile with a profile centre point 51. Theprofile of the cutter teeth 53 is derived from the cylindrical pinionwhich can mesh with the crown wheel. The profile centre point 51 lies ona circle in the middle 50 of the cutter 43. In order to give the cutterteeth 53 as much stability as possible, the outer circumference 54 ofthe cutter 43 is kept as large as possible in this example, and thecutter 43 can also have the narrower contour 57.

The line 55 indicates the tangent to the surface 40 of the workpiece 38to be worked when the cutter 43 is positioned at right angles to saidsurface 40 to be worked. If there is a changeover to off-centre cuttingwhile the position of the rotation shaft of the cutter is retained, thetangent to the surface of the workpiece 38 will run in accordance withthe line 56 after the cutter 43 has been placed off centre. This is anundesirable situation, because the cutter teeth 53 will be loadedasymmetrically as a result, and because uneven wear will occur. In thecase of the method according to the present invention, in order to avoidthis undesirable situation, the rotation shaft of the cutter 43 isrotated from the position 45, about centre point 51 of the cutter teethin engagement with the workpiece 38, to the position 46. This makes thecentre point 51, the centre point 5 of the cutter 43 situated on theaxis of rotation of the cutter 43, and the axis of rotation 22 of thecrown wheel to be produced go into one plane. This plane is indicated by"l" in FIG. 6. The position of the cutter 43 obtained is indicated inFIG. 6 by position 44. This suitable positioning means that the cutter43 works the workpiece uniformly and that symmetrical wear occurs in theprocess.

In the case of the method according to the invention indicateddiagrammatically in FIG. 8, the above-described positioning of thecutter 43 is maintained during working of the workpiece 38. This meansthat the cutter 43 is in the position indicated by 59 at the start ofworking, which position corresponds to the position 44 shown in FIG. 6,and is in the position indicated by 61 on the completion of working. Itcan be seen clearly from FIG. 8 that during working of the outer surface40 from the large diameter to the small diameter the centre point 51 ofthe cutter teeth which are in engagement with the workpiece moves alongthe axis of rotation 41 of the cylindrical pinion which can mesh withthe crown wheel to be produced. As already mentioned, the centre point 5of the cutter 43, the centre point 51 of the teeth of the cutter 43 andthe axis of rotation 22 of the workpiece 38 remain in one plane duringthe generating process. In the position 59 of the cutter 43 this planeis 58, and in the position 61 this plane is 60.

This means that in the case of the method followed here the cutter 43carries out a rotation about the centre point 51 of the meshing cutterteeth during the movement from large diameter to small diameter. Inorder to achieve this, the relative movement between cutter 43 andworkpiece 38 must acquire an additional rotation which is comparable tothe differential movement during working of helical toothed cylindricalpinions. When the device shown in FIGS. 4 and 5 is used, this means thatit is necessary to rotate the cutter head 36 about the axis of rotation37 during production of the toothing on the workpiece.

FIG. 9 shows diagrammatically a method according to the invention forthe production of a crown wheel which can mesh with a cylindrical pinion(not shown) which is placed off centre. In the case of this method,unlike the method described with reference to FIG. 8, the centre point 5of cutter 43 situated on the rotation shaft of the cutter, the centrepoint 51 of the meshing cutter teeth, and the axis of rotation 22 of theworkpiece 38 during working of the workpiece lie only in one position,for example in the centre of the outer surface 40 (position 63 of thecutter 43) in one plane 65. Furthermore, it can be seen clearly fromFIG. 9 that the cutter 43 is moved from position 62 to position 64 in astraight line, parallel to the axis of rotation 41 of the cylindricalpinion which can mesh with the crown wheel. The result of this is thatin positions 62 and 64, for example, the cutter is not in a positionaccurately at right angles to the surface of the workpiece to be worked,but on average it is in such a position, so that the wear on the cutterremains acceptable. Due to the fact that the cutter moves in one plane,the rotation of the cutter head 36 about the axis of rotation 37 in thecase of the method described with reference to FIG. 8 can be dispensedwith.

I claim:
 1. Method of producing a conical or cup-shaped crown wheel which can mesh with a characteristic cylindrical pinion having a tooth profile and a pinion center, which pinion has an axis of rotation which intersects or crosses the axis of rotation of the crown wheel with the inclusion of an angle α deviating from 90 degrees, the method comprising a generating process in which a tool and a workpiece rotate at a constant ratio of rotation speeds and are moved relative to each other in such a manner that the tool continuously works the workpiece, wherein the tool comprises a disc which is rotatable about its rotation shaft and has machining elements disposed on its circumference whose cutting edges lie in the surface of a profile which extends essentially in the form of helical ribs at a pitch angle γ around the circumference of the disc, said ribs forming in each cross-sectional plane perpendicular to the helical direction of the ribs a profile section having the shape of a number of adjacent cutting teeth which have a profile center point, the shape of the profile section being based on the basic geometry of the characteristic cylindrical pinion and said profile center point being identical to said pinion center and lying on a circle in a plane perpendicular to the rotation shaft of the tool, which circle has a circle center point lying on the rotation shaft of the tool, which circle center point forms a tool center point of the tool, wherein the tool is positioned relative to the workpiece in such a way that at least for one point during the working of the workpiece the profile center point of the adjacent cutting teeth which are working the workpiece lies essentially in a first plane defined by the axis of rotation of the workpiece and the tool center point, and wherein the tool is moved with respect to the workpiece throughout the working of the workpiece such that the profile center point of the adjacent cutting teeth which are working the workpiece is moved in a first straight line which intersects or crosses the axis of rotation of the workpiece with the inclusion of the angle α.
 2. Method according to claim 1, wherein the tool is positioned relative to the workpiece in such a way that throughout the working of the workpiece the profile center point of the adjacent cutting teeth which are working the workpiece lies essentially in said first plane.
 3. Method according to claim 1, wherein said characteristic cylindrical pinion has a tooth angle, and wherein during the working of the workpiece the tool is positioned relative to the workpiece through pivotal movement of the rotation shaft of the tool about a pivot axis which is directed at right angles to the rotation shaft of the tool and the first straight line, such that the helical ribs of the tool are brought into an angle with said first plane corresponding essentially to the tooth angle of the characteristic cylindrical pinion.
 4. Method according to claim 1, wherein during the generating process the tool center point is moved along a second straight line parallel to said first straight line.
 5. Method according to claim 1, wherein said first plane forms an angle β with a second plane, which second plane comprises the axis of rotation of the workpiece and forms a right angle with the rotation shaft of the tool, and wherein the rotation shaft of the tool forms an angle δ with a third plane extending at right angles to the axis of rotation of the workpiece, and wherein the following applies for the angle β between the first plane and the second plane:β=arctan (-sin α·tan γ), and wherein prior to the generating process the angle δ is set in such a way that the following applies:δ=arctan (cos α·tan γ·cos β).
 6. Method according to claim 1, wherein said first plane forms an angle β with a second plane, which second plane comprises the axis of rotation of the workpiece and forms a right angle with the rotation shaft of the tool, and wherein the rotation shaft of the tool forms an angle δ with a third plane extending at right angles to the axis of rotation of the workpiece, and wherein the following applies for the angle β between the first plane and the second plane: β=arctan (-sin α·tan γ), and wherein prior to the generating process the angle δ is set in such a way that the following applies: δ=(γ·cosα). 